Most Important Aspect of Headphones

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When we talk about headphones, we talk about comfort, bass, sound leakage, and so on and so forth. What components do you think are the most important?
I want you to make a list from the most to the least valuable factors. I'll give you my list as an example, but you don't have to take into account the same factors that I did. Feel free to add or subtract whatever you please. (I think I missed a lot of stuff, anyways.)

Rather than get into listing the typical culprits, I want to touch on a couple things that people may overlook if they're stuck in the mindset of singular parameters.
a) Tonal balance: In general, the overall shape of the frequency response. A bit more specifically, transitioning from region to region (any area, not just the fuzzy concepts of bass/mids/treble). For most people, FR is the first (and easiest) thing to notice with headphones.
b) Distortion and phase error: harmonic and intermodular distortion, diaphragm breakup, and phase deviations are all aspects that move the reproduced sound away from the input signal in a manner that isn't (directly) related to the frequency response. Being able to decipher things in this category is more difficult than tonal changes.
c) Channel matching: mismatched sides, even by one or two dB, can create a very large change in sonic perception and is often mistaken as a poor FR or distortion.